The present invention relates to a method of damping control oscillations of a motor vehicle traction control system operating by brake intervention on the low-adhesion wheel.
Electronic traction systems, in general, provide for brake intervention on the wheel first tending to spin, the so-called low-adhesion wheel as described, for example, in the article "Ausgebremst" in the Auto-Motor-Sport Journal of Aug. 2, 1986, p. 34. Such situations arise, in particular, where different friction coefficients are present for the driving wheels. Because of the one-sided brake intervention on the low-adhesion wheel, a corresponding traction-increasing lock torque is transmitted to the high-adhesion wheel. Consequently, such systems replace the automatic differential lock which is also used for this purpose.
When such an electronic traction system is employed, the danger exists that the traction control may exhibit undesirable control oscillation behavior which can, for example, lead to the so-called pull-away tramping effect or to so-called drive train oscillations. The pull-away tramping effect is based on the two driving wheels frequently interchanging low-adhesion wheel and high-adhesion wheel behavior. It is a particular danger where low friction coefficients are present at both driving wheels because, as a rule, the friction coefficient and the wheel loads never agree completely and the two wheels do not, therefore, start to spin simultaneously. The driving wheel which spins first then represents the low-adhesion wheel for a traction control system but the opposite high-adhesion wheel would usually also spin a short time later, even without low-wheel brake intervention. This tendency is further amplified by the build-up of pressure for traction control purposes on the low-adhesion wheel. This can now lead to the high-adhesion wheel spinning, whereas the low-adhesion wheel slip is reduced to a very low value.
Consequently, the low-adhesion wheel to be controlled in the known method is now the previous high-adhesion wheel instead of the previous low-adhesion wheel. Pressure is then reduced at the new high-adhesion wheel whereas pressure is built up at the new low-adhesion wheel. Because of the low friction coefficient, however, the new high-adhesion wheel cannot, in turn, offer the necessary supporting torque and it spins once again. If countermeasures are not taken, this procedure occurs cyclically and causes high-frequency wheel oscillations and a saw-tooth-like build-up and reduction of pressure on alternate sides. This is referred to as pull-away tramping.
A further reason for undesirable traction control oscillations in these known systems is due, in large part, to an exponential variation of the slip/friction coefficient characteristic which exhibits a maximum. Such a variation appears, for example, in the case of ice with an adhesion maximum or on wet grass. The result of this characteristic curve is that a driving wheel which is initially in a slip range with a high friction coefficient can deliver a relatively large traction torque until it leaves the limiting friction condition and enters the sliding friction condition. The adhesion is simultaneously reduced due to the increase in slip so that traction control is initiated and brake pressure is built up in this wheel, which is treated as the low-adhesion wheel, in order to control the traction. This reduces the slip again. Because of the decreasing slip, however, the friction coefficient is simultaneously increased once again. The low-adhesion wheel is therefore braked even more strongly so that it runs almost without slip. This causes the pressure to be reduced once again until the low-adhesion wheel again begins to spin. The drive train is excited to oscillation by repetition of this overshoot in the traction control and driving comfort can be disturbingly impaired.
DE 32 36 366 A1 describes a device for utilizing the possible adhesion coefficients of two vehicle wheels driven by a differential gear. The brake intervention on the more rapidly rotating low-adhesion wheel takes place with a continually increasing braking effect when the rotational speed difference between the driving wheels exceeds a specified threshold value. This continues until the rotational speed of the low-adhesion wheel has again fallen below the threshold value and both wheels have, therefore, approximately the same rotational speed. After the low-adhesion wheel has fallen below the threshold value, the low-adhesion wheel brake pressure is reduced in accordance with a specified pressure gradient. If the rotational speed of the previous high-adhesion wheel now increases above the rotational speed of the low-adhesion wheel by the specified threshold amount before the reduction in brake pressure at the low-adhesion wheel has ended, a brake intervention to control traction is initiated at the previous high-adhesion wheel while, at the same time, brake pressure is still present at the previous low-adhesion wheel.
DE 27 56 192 A1 describes a device for controlling the torque transmitted to the driving wheels of a motor vehicle via a differential gear. Magnetic valves which effect the activation and deactivation of the driving wheel brakes are triggered so that braking is effected on that driving wheel whose acceleration exceeds an acceleration threshold value when the speed of the driving wheels is above a specified speed threshold value and when the other driving wheel is not accelerated with an acceleration value located above the acceleration threshold value. A release of the brake is effected on that driving wheel on which the brake has acted when its rotational speed decreases and/or the other driving wheel accelerates and as soon as the speed difference between the two driving wheels is smaller than a specified speed difference threshold value. Only one magnetic valve at a time can be switched on for brake activation. An interchange from a low-adhesion wheel whose traction was previously controlled by a brake intervention to traction control on the previous high-adhesion wheel by corresponding brake action has already taken place, inter alia, when the acceleration of the previous high-adhesion wheel exceeds the specified acceleration threshold value. The brake on the previous low-adhesion wheel is then simultaneously deactivated and that on the previous high-adhesion wheel is activated. This takes place independently of whether the rotational speed at the previous high-adhesion wheel is or is not greater than that at the previous low-adhesion wheel.
It is an object of the present invention to provide a method which substantially avoids disturbing control oscillations of an electronic traction system, in particular pull-away tramping and/or drive train oscillation effects.
The foregoing object has been achieved in accordance with the present invention by a method in which an interchange of the low-adhesion wheel to be controlled from one driving wheel to an opposite driving wheel occurs, at the earliest, when a rotational speed of a previous high-adhesion wheel is greater by a specified value than the rotational speed of the previous low-adhesion wheel and when there is no longer any brake pressure at the previous low-adhesion wheel or a variation with time of a magnitude of slip difference between the driving wheels and, raising by a specified amount, when oscillation behavior of the same is recognized, an activation threshold value for activating a traction-increasing brake intervention on the low-adhesion wheel.
Thereby, an excessively frequent interchange of low-adhesion wheel and high-adhesion wheel between the driving wheels is prevented by the method of the present invention. A counteraction for the pull-away tramping effect, in particular, occurs because such interchange does not take place as soon as the high-adhesion wheel slip becomes equal to the low-adhesion wheel slip but only when the former is greater than the low-adhesion wheel slip by a specified amount and when, in addition, there is no longer any brake pressure present at the low-adhesion wheel from a previous traction control procedure. Furthermore, the brake pressure at the previous low-adhesion wheel is always reduced completely before a new traction control procedure begins at the previous high-adhesion wheel as the potential new low-adhesion wheel in accordance with the present invention.
A mode of operation according to another embodiment of the present invention prevents drive train oscillation behavior because the variation with time of the magnitude of the driving wheel slip difference, which is used as the control parameter, is recorded. If oscillation behavior of this driving wheel slip difference is recognized, the threshold value for activating a particular traction control procedure is raised. This reduces the frequency of alternating activation, and deactivation of the traction control system and the control therefore is made smoother.
A low-adhesion/high-adhesion wheel interchange is not permitted in accordance with the present invention until a specified hysteresis period has elapsed after a previous interchange and provides additional damping of the pull-away tramping effect. Because this effect is more disturbing at relatively high speeds than at low speeds, the hysteresis period is preferably fixed as a function of the vehicle speed. Especially in the case of active traction control, such a low-adhesion/high-adhesion wheel interchange is only permitted when the vehicle speed is below a specified value. This counteracts the tramping on alternate sides at relatively high speeds whereas, in the low speed range in the case of a friction coefficient split, the pull-away traction is improved.
A further aspect of the present invention makes available a variable increase, which is matched to the situation, in the activation threshold for the traction control by setting the new threshold value in each case to the last maximum recognized in the slip difference curve plus a fixed offset value. In contrast to the selection of a static activation threshold which is much too high for certain friction coefficient relationships, this approach achieves a variable, dynamic increase in the activation slip threshold, which is matched in the best possible manner to the different driving surface conditions so that even the worst possible case is also covered.
An offset value is preferred which is larger than the difference between, on one hand, the activation threshold value and, on the other hand, a deactivation threshold value which is used for deactivating a traction control procedure. In the case of an increase in the activation threshold value, therefore, the magnitude of the actual slip difference is below the deactivation threshold so that the traction control initiates a reduction in pressure, and the low-adhesion wheel slip can increase further in order to counteract an increase in the drive train oscillation.
A further feature of the present invention makes available an advantageous way of reliably recognizing drive train oscillation behavior.